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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RUNTIME-CONFIG-RESOURCE"
>18.4. Resource Consumption</A
></H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-MEMORY"
>18.4.1. Memory</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-SHARED-BUFFERS"
></A
><TT
CLASS="VARNAME"
>shared_buffers</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Sets the amount of memory the database server uses for shared
        memory buffers.  The default is typically 32 megabytes
        (<TT
CLASS="LITERAL"
>32MB</TT
>), but might be less if your kernel settings will
        not support it (as determined during <SPAN
CLASS="APPLICATION"
>initdb</SPAN
>).
        This setting must be at least 128 kilobytes.  (Non-default
        values of <TT
CLASS="SYMBOL"
>BLCKSZ</TT
> change the minimum.)  However,
        settings significantly higher than the minimum are usually needed
        for good performance.  This parameter can only be set at server start.
       </P
><P
>        If you have a dedicated database server with 1GB or more of RAM, a
        reasonable starting value for <TT
CLASS="VARNAME"
>shared_buffers</TT
> is 25%
        of the memory in your system.  There are some workloads where even
        large settings for <TT
CLASS="VARNAME"
>shared_buffers</TT
> are effective, but
        because <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> also relies on the
        operating system cache, it is unlikely that an allocation of more than
        40% of RAM to <TT
CLASS="VARNAME"
>shared_buffers</TT
> will work better than a
        smaller amount.  Larger settings for <TT
CLASS="VARNAME"
>shared_buffers</TT
>
        usually require a corresponding increase in
        <TT
CLASS="VARNAME"
>checkpoint_segments</TT
>, in order to spread out the
        process of writing large quantities of new or changed data over a
        longer period of time.
       </P
><P
>        On systems with less than 1GB of RAM, a smaller percentage of RAM is
        appropriate, so as to leave adequate space for the operating system.
        Also, on Windows, large values for <TT
CLASS="VARNAME"
>shared_buffers</TT
>
        aren't as effective.  You may find better results keeping the setting
        relatively low and using the operating system cache more instead.  The
        useful range for <TT
CLASS="VARNAME"
>shared_buffers</TT
> on Windows systems
        is generally from 64MB to 512MB.
       </P
><P
>        Increasing this parameter might cause <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
        to request more <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> shared
        memory than your operating system's default configuration
        allows. See <A
HREF="kernel-resources.html#SYSVIPC"
>Section 17.4.1</A
> for information on how to
        adjust those parameters, if necessary.
       </P
></DD
><DT
><A
NAME="GUC-TEMP-BUFFERS"
></A
><TT
CLASS="VARNAME"
>temp_buffers</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Sets the maximum number of temporary buffers used by each database
        session.  These are session-local buffers used only for access to
        temporary tables.  The default is eight megabytes
        (<TT
CLASS="LITERAL"
>8MB</TT
>).  The setting can be changed within individual
        sessions, but only before the first use of temporary tables
        within the session; subsequent attempts to change the value will
        have no effect on that session.
       </P
><P
>        A session will allocate temporary buffers as needed up to the limit
        given by <TT
CLASS="VARNAME"
>temp_buffers</TT
>.  The cost of setting a large
        value in sessions that do not actually need many temporary
        buffers is only a buffer descriptor, or about 64 bytes, per
        increment in <TT
CLASS="VARNAME"
>temp_buffers</TT
>.  However if a buffer is
        actually used an additional 8192 bytes will be consumed for it
        (or in general, <TT
CLASS="SYMBOL"
>BLCKSZ</TT
> bytes).
       </P
></DD
><DT
><A
NAME="GUC-MAX-PREPARED-TRANSACTIONS"
></A
><TT
CLASS="VARNAME"
>max_prepared_transactions</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Sets the maximum number of transactions that can be in the
        <SPAN
CLASS="QUOTE"
>"prepared"</SPAN
> state simultaneously (see <A
HREF="sql-prepare-transaction.html"
>PREPARE TRANSACTION</A
>).
        Setting this parameter to zero (which is the default)
        disables the prepared-transaction feature.
        This parameter can only be set at server start.
       </P
><P
>        If you are not planning to use prepared transactions, this parameter
        should be set to zero to prevent accidental creation of prepared
        transactions.  If you are using prepared transactions, you will
        probably want <TT
CLASS="VARNAME"
>max_prepared_transactions</TT
> to be at
        least as large as <A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
>, so that every
        session can have a prepared transaction pending.
       </P
><P
>        Increasing this parameter might cause <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
        to request more <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> shared
        memory than your operating system's default configuration
        allows. See <A
HREF="kernel-resources.html#SYSVIPC"
>Section 17.4.1</A
> for information on how to
        adjust those parameters, if necessary.
       </P
><P
>        When running a standby server, you must set this parameter to the
        same or higher value than on the master server. Otherwise, queries
        will not be allowed in the standby server.
       </P
></DD
><DT
><A
NAME="GUC-WORK-MEM"
></A
><TT
CLASS="VARNAME"
>work_mem</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Specifies the amount of memory to be used by internal sort operations
        and hash tables before writing to temporary disk files. The value
        defaults to one megabyte (<TT
CLASS="LITERAL"
>1MB</TT
>).
        Note that for a complex query, several sort or hash operations might be
        running in parallel; each operation will be allowed to use as much memory
        as this value specifies before it starts to write data into temporary
        files. Also, several running sessions could be doing such operations
        concurrently.  Therefore, the total memory used could be many
        times the value of <TT
CLASS="VARNAME"
>work_mem</TT
>; it is necessary to
        keep this fact in mind when choosing the value. Sort operations are
        used for <TT
CLASS="LITERAL"
>ORDER BY</TT
>, <TT
CLASS="LITERAL"
>DISTINCT</TT
>, and
        merge joins.
        Hash tables are used in hash joins, hash-based aggregation, and
        hash-based processing of <TT
CLASS="LITERAL"
>IN</TT
> subqueries.
       </P
></DD
><DT
><A
NAME="GUC-MAINTENANCE-WORK-MEM"
></A
><TT
CLASS="VARNAME"
>maintenance_work_mem</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Specifies the maximum amount of memory to be used by maintenance
        operations, such as <TT
CLASS="COMMAND"
>VACUUM</TT
>, <TT
CLASS="COMMAND"
>CREATE
        INDEX</TT
>, and <TT
CLASS="COMMAND"
>ALTER TABLE ADD FOREIGN KEY</TT
>.  It defaults
        to 16 megabytes (<TT
CLASS="LITERAL"
>16MB</TT
>).  Since only one of these
        operations can be executed at a time by a database session, and
        an installation normally doesn't have many of them running
        concurrently, it's safe to set this value significantly larger
        than <TT
CLASS="VARNAME"
>work_mem</TT
>.  Larger settings might improve
        performance for vacuuming and for restoring database dumps.
       </P
><P
>        Note that when autovacuum runs, up to
        <A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
> times this memory may be
        allocated, so be careful not to set the default value too high.
       </P
></DD
><DT
><A
NAME="GUC-MAX-STACK-DEPTH"
></A
><TT
CLASS="VARNAME"
>max_stack_depth</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Specifies the maximum safe depth of the server's execution stack.
        The ideal setting for this parameter is the actual stack size limit
        enforced by the kernel (as set by <TT
CLASS="LITERAL"
>ulimit -s</TT
> or local
        equivalent), less a safety margin of a megabyte or so.  The safety
        margin is needed because the stack depth is not checked in every
        routine in the server, but only in key potentially-recursive routines
        such as expression evaluation.  The default setting is two
        megabytes (<TT
CLASS="LITERAL"
>2MB</TT
>), which is conservatively small and
        unlikely to risk crashes.  However, it might be too small to allow
        execution of complex functions.  Only superusers can change this
        setting.
       </P
><P
>        Setting <TT
CLASS="VARNAME"
>max_stack_depth</TT
> higher than
        the actual kernel limit will mean that a runaway recursive function
        can crash an individual backend process.  On platforms where
        <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can determine the kernel limit,
        the server will not allow this variable to be set to an unsafe
        value.  However, not all platforms provide the information,
        so caution is recommended in selecting a value.
       </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-KERNEL"
>18.4.2. Kernel Resource Usage</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-MAX-FILES-PER-PROCESS"
></A
><TT
CLASS="VARNAME"
>max_files_per_process</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>        Sets the maximum number of simultaneously open files allowed to each
        server subprocess. The default is one thousand files. If the kernel is enforcing
        a safe per-process limit, you don't need to worry about this setting.
        But on some platforms (notably, most BSD systems), the kernel will
        allow individual processes to open many more files than the system
        can actually support if many processes all try to open
        that many files. If you find yourself seeing <SPAN
CLASS="QUOTE"
>"Too many open
        files"</SPAN
> failures, try reducing this setting.
        This parameter can only be set at server start.
       </P
></DD
><DT
><A
NAME="GUC-SHARED-PRELOAD-LIBRARIES"
></A
><TT
CLASS="VARNAME"
>shared_preload_libraries</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        This variable specifies one or more shared libraries
        to be preloaded at server start. For example,
        <TT
CLASS="LITERAL"
>'$libdir/mylib'</TT
> would cause
        <TT
CLASS="LITERAL"
>mylib.so</TT
> (or on some platforms,
        <TT
CLASS="LITERAL"
>mylib.sl</TT
>) to be preloaded from the installation's
        standard library directory.
        All library names are converted to lower case unless double-quoted.
        If more than one library is to be loaded, separate their names
        with commas.  This parameter can only be set at server start.
       </P
><P
>        <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> procedural language
        libraries can be preloaded in this way, typically by using the
        syntax <TT
CLASS="LITERAL"
>'$libdir/plXXX'</TT
> where
        <TT
CLASS="LITERAL"
>XXX</TT
> is <TT
CLASS="LITERAL"
>pgsql</TT
>, <TT
CLASS="LITERAL"
>perl</TT
>,
        <TT
CLASS="LITERAL"
>tcl</TT
>, or <TT
CLASS="LITERAL"
>python</TT
>.
       </P
><P
>        By preloading a shared library, the library startup time is avoided
        when the library is first used.  However, the time to start each new
        server process might increase slightly, even if that process never
        uses the library.  So this parameter is recommended only for
        libraries that will be used in most sessions.
       </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>        On Windows hosts, preloading a library at server start will not reduce
        the time required to start each new server process; each server process
        will re-load all preload libraries.  However, <TT
CLASS="VARNAME"
>shared_preload_libraries
        </TT
> is still useful on Windows hosts because some shared libraries may
        need to perform certain operations that only take place at postmaster start
        (for example, a shared library may need to reserve lightweight locks
        or shared memory and you can't do that after the postmaster has started).
       </P
></BLOCKQUOTE
></DIV
><P
>        If a specified library is not found,
        the server will fail to start.
       </P
><P
>        Every  PostgreSQL-supported library has a <SPAN
CLASS="QUOTE"
>"magic
        block"</SPAN
> that is checked to guarantee compatibility.
        For this reason, non-PostgreSQL libraries cannot be
        loaded in this way.
       </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-VACUUM-COST"
>18.4.3. Cost-based Vacuum Delay</A
></H2
><P
>      During the execution of <A
HREF="sql-vacuum.html"
>VACUUM</A
>
      and <A
HREF="sql-analyze.html"
>ANALYZE</A
>
      commands, the system maintains an
      internal counter that keeps track of the estimated cost of the
      various I/O operations that are performed.  When the accumulated
      cost reaches a limit (specified by
      <TT
CLASS="VARNAME"
>vacuum_cost_limit</TT
>), the process performing
      the operation will sleep for a short period of time, as specified by
      <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
>. Then it will reset the
      counter and continue execution.
     </P
><P
>      The intent of this feature is to allow administrators to reduce
      the I/O impact of these commands on concurrent database
      activity. There are many situations where it is not
      important that maintenance commands like
      <TT
CLASS="COMMAND"
>VACUUM</TT
> and <TT
CLASS="COMMAND"
>ANALYZE</TT
> finish
      quickly; however, it is usually very important that these
      commands do not significantly interfere with the ability of the
      system to perform other database operations. Cost-based vacuum
      delay provides a way for administrators to achieve this.
     </P
><P
>      This feature is disabled by default for manually issued
      <TT
CLASS="COMMAND"
>VACUUM</TT
> commands. To enable it, set the
      <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> variable to a nonzero
      value.
     </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-VACUUM-COST-DELAY"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         The length of time, in milliseconds, that the process will sleep
         when the cost limit has been exceeded.
         The default value is zero, which disables the cost-based vacuum
         delay feature.  Positive values enable cost-based vacuuming.
         Note that on many systems, the effective resolution
         of sleep delays is 10 milliseconds; setting
         <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> to a value that is
         not a multiple of 10 might have the same results as setting it
         to the next higher multiple of 10.
        </P
><P
>         When using cost-based vacuuming, appropriate values for
         <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> are usually quite small, perhaps
         10 or 20 milliseconds.  Adjusting vacuum's resource consumption
         is best done by changing the other vacuum cost parameters.
        </P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-PAGE-HIT"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_page_hit</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         The estimated cost for vacuuming a buffer found in the shared buffer
         cache. It represents the cost to lock the buffer pool, lookup
         the shared hash table and scan the content of the page. The
         default value is one.
        </P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-PAGE-MISS"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_page_miss</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         The estimated cost for vacuuming a buffer that has to be read from
         disk.  This represents the effort to lock the buffer pool,
         lookup the shared hash table, read the desired block in from
         the disk and scan its content. The default value is 10.
        </P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-PAGE-DIRTY"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_page_dirty</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         The estimated cost charged when vacuum modifies a block that was
         previously clean. It represents the extra I/O required to
         flush the dirty block out to disk again. The default value is
         20.
        </P
></DD
><DT
><A
NAME="GUC-VACUUM-COST-LIMIT"
></A
><TT
CLASS="VARNAME"
>vacuum_cost_limit</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         The accumulated cost that will cause the vacuuming process to sleep.
         The default value is 200.
        </P
></DD
></DL
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>       There are certain operations that hold critical locks and should
       therefore complete as quickly as possible.  Cost-based vacuum
       delays do not occur during such operations.  Therefore it is
       possible that the cost accumulates far higher than the specified
       limit.  To avoid uselessly long delays in such cases, the actual
       delay is calculated as <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> *
       <TT
CLASS="VARNAME"
>accumulated_balance</TT
> /
       <TT
CLASS="VARNAME"
>vacuum_cost_limit</TT
> with a maximum of
       <TT
CLASS="VARNAME"
>vacuum_cost_delay</TT
> * 4.
      </P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER"
>18.4.4. Background Writer</A
></H2
><P
>      There is a separate server
      process called the <I
CLASS="FIRSTTERM"
>background writer</I
>, whose function
      is to issue writes of <SPAN
CLASS="QUOTE"
>"dirty"</SPAN
> (new or modified) shared
      buffers.  It writes shared buffers so server processes handling
      user queries seldom or never need to wait for a write to occur.
      However, the background writer does cause a net overall
      increase in I/O load, because while a repeatedly-dirtied page might
      otherwise be written only once per checkpoint interval, the
      background writer might write it several times as it is dirtied
      in the same interval.  The parameters discussed in this subsection
      can be used to tune the behavior for local needs.
     </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-BGWRITER-DELAY"
></A
><TT
CLASS="VARNAME"
>bgwriter_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         Specifies the delay between activity rounds for the
         background writer.  In each round the writer issues writes
         for some number of dirty buffers (controllable by the
         following parameters).  It then sleeps for <TT
CLASS="VARNAME"
>bgwriter_delay</TT
>
         milliseconds, and repeats.  The default value is 200 milliseconds
         (<TT
CLASS="LITERAL"
>200ms</TT
>). Note that on many systems, the effective
         resolution of sleep delays is 10 milliseconds; setting
         <TT
CLASS="VARNAME"
>bgwriter_delay</TT
> to a value that is not a multiple of
         10 might have the same results as setting it to the next higher
         multiple of 10.  This parameter can only be set in the
         <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
        </P
></DD
><DT
><A
NAME="GUC-BGWRITER-LRU-MAXPAGES"
></A
><TT
CLASS="VARNAME"
>bgwriter_lru_maxpages</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         In each round, no more than this many buffers will be written
         by the background writer.  Setting this to zero disables
         background writing (except for checkpoint activity).
         The default value is 100 buffers.
         This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
         file or on the server command line.
        </P
></DD
><DT
><A
NAME="GUC-BGWRITER-LRU-MULTIPLIER"
></A
><TT
CLASS="VARNAME"
>bgwriter_lru_multiplier</TT
> (<TT
CLASS="TYPE"
>floating point</TT
>)</DT
><DD
><P
>         The number of dirty buffers written in each round is based on the
         number of new buffers that have been needed by server processes
         during recent rounds.  The average recent need is multiplied by
         <TT
CLASS="VARNAME"
>bgwriter_lru_multiplier</TT
> to arrive at an estimate of the
         number of buffers that will be needed during the next round.  Dirty
         buffers are written until there are that many clean, reusable buffers
         available.  (However, no more than <TT
CLASS="VARNAME"
>bgwriter_lru_maxpages</TT
>
         buffers will be written per round.)
         Thus, a setting of 1.0 represents a <SPAN
CLASS="QUOTE"
>"just in time"</SPAN
> policy
         of writing exactly the number of buffers predicted to be needed.
         Larger values provide some cushion against spikes in demand,
         while smaller values intentionally leave writes to be done by
         server processes.
         The default is 2.0.
         This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
         file or on the server command line.
        </P
></DD
></DL
></DIV
><P
>      Smaller values of <TT
CLASS="VARNAME"
>bgwriter_lru_maxpages</TT
> and
      <TT
CLASS="VARNAME"
>bgwriter_lru_multiplier</TT
> reduce the extra I/O load
      caused by the background writer, but make it more likely that server
      processes will have to issue writes for themselves, delaying interactive
      queries.
     </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-RESOURCE-ASYNC-BEHAVIOR"
>18.4.5. Asynchronous Behavior</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-EFFECTIVE-IO-CONCURRENCY"
></A
><TT
CLASS="VARNAME"
>effective_io_concurrency</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
>         Sets the number of concurrent disk I/O operations that
         <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> expects can be executed
         simultaneously.  Raising this value will increase the number of I/O
         operations that any individual <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> session
         attempts to initiate in parallel.  The allowed range is 1 to 1000,
         or zero to disable issuance of asynchronous I/O requests. Currently,
         this setting only affects bitmap heap scans.
        </P
><P
>         A good starting point for this setting is the number of separate
         drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
         database.  (For RAID 5 the parity drive should not be counted.)
         However, if the database is often busy with multiple queries issued in
         concurrent sessions, lower values may be sufficient to keep the disk
         array busy.  A value higher than needed to keep the disks busy will
         only result in extra CPU overhead.
        </P
><P
>         For more exotic systems, such as memory-based storage or a RAID array
         that is limited by bus bandwidth, the correct value might be the
         number of I/O paths available.  Some experimentation may be needed
         to find the best value.
        </P
><P
>         Asynchronous I/O depends on an effective <CODE
CLASS="FUNCTION"
>posix_fadvise</CODE
>
         function, which some operating systems lack.  If the function is not
         present then setting this parameter to anything but zero will result
         in an error.  On some operating systems (e.g., Solaris), the function
         is present but does not actually do anything.
        </P
></DD
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